While signals within telecommunications and data communications networks have been traditionally exchanged by transmitting electrical signals via electrically conductive lines, an alternative medium of data exchange is the transmission of optical signals through optical fibers. Information is exchanged in the form of modulations of laser-produced light. The equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the design of optical switches for use in telecommunications and data communications networks is problematic. As a result, switching requirements within a network that transmits optical signals is often satisfied by converting the optical signals to electrical signals at the inputs of a switching network, then reconverting the electrical signals to optical signals at the outputs of the switching network.
Recently, reliable optical switching systems have been developed. U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention, describes a switching matrix that may be used for routing optical signals from one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers. Another such matrix of switching elements is described in U.S. Pat. No. 4,988,157 to Jackel et al. An isolated switching element 10 is shown in FIG. 1, while a 4.times.4 matrix 32 of switching elements is shown in FIG. 2. The optical switch of FIG. 1 is formed on a substrate. The substrate may be a silicon substrate, but other materials may be used. The optical switch 10 includes planar waveguides defined by a lower cladding layer 14, a core 16, and an upper cladding layer, not shown. The core is primarily silicon dioxide, but with other materials that achieve a desired index of refraction for the core. The cladding layers should be formed of a material having a refractive index that is substantially different from the refractive index of the core material, so that optical signals are guided along the waveguides.
The material core 16 is patterned to form an input waveguide 20 and an output waveguide 26 of a first optical path and to define a second input waveguide 24 and a second output waveguide 22 of a second optical path. The upper cladding layer is then deposited over the patterned core material. A gap 28 is formed by etching a trench through the core material and the two cladding layers to the substrate. The waveguides intersect the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the location 30 aligned with the waveguides is filled with a vapor or gas. Thus, TIR diverts light from the input waveguide 20 to the output waveguide 22, unless an index-matching fluid resides within the location 30 between the aligned waveguides 20 and 26. The trench 28 is positioned with respect to the four waveguides such that one sidewall of the trench passes through or slightly offset from the intersection of the axes of the waveguides.
The above-identified patent to Fouquet et al. describes a number of alternative approaches to switching the switching element 10 between a transmissive state and a reflective state. The element includes at least one heater that can be used to manipulate fluid within the gap 28. One approach is illustrated in FIG. 1. The switching element 10 includes two microheaters 50 and 52 that control the position of a bubble within the fluid-containing gap. The fluid within the gap has a refractive index that is close to the refractive index of the core material 16 of the four waveguides 20-26. Fluid fill-holes 54 and 56 may be used to provide a steady supply of fluid, but this is not critical. In the operation of the switching element, one of the heaters 50 and 52 is brought to a temperature sufficiently high to form a gas bubble. Once formed, the bubble can be maintained in position with a reduced current to the heater. In FIG. 1, the bubble is positioned at the location 30 of the intersection of the four waveguides. Consequently, an input signal along the waveguide 20 will encounter a refractive index mismatch upon reaching the gap 28. This places the switching element in a reflecting state, causing the optical signal along the waveguide 20 to be redirected to the output waveguide 22. However, even in the reflecting state, the second input waveguide 24 is not in communication with the output waveguide 26.
If the heater 50 at location 30 is deactivated and the second heater 52 is activated, the bubble will be attracted to the off-axis heater 52. This allows index-matching fluid to fill the location 30 at the intersection of the waveguides 20-26. The switching element 10 is then in a transmitting state, since the input waveguide 20 is optically coupled to the collinear waveguide 26.
In the 4.times.4 matrix 32 of FIG. 2, any of the four input waveguides 34, 36, 38 and 40 may be optically coupled to any one of the four output waveguides 42, 44, 46 and 48. The switching matrix is sometimes referred to as a "non-blocking" matrix, since any free input fiber can be connected to any free output fiber regardless of which connections have already been made through the switching matrix. Each of the sixteen optical switches has a gap that causes TIR in the absence of a fluid at the location between collinear waveguides, but collinear waveguides of a particular waveguide path are optically coupled when the locations between the waveguides are filled with the fluid. Trenches that are in the transmissive state are represented by fine lines that extend at an angle through the intersections of the optical waveguides in the matrix. On the other hand, trenches of switching elements in a reflecting state are represented by broad lines through points of intersection.
In FIGS. 1 and 2, the input waveguide 20 is in optical communication with the output waveguide 22, as a result of TIR at the empty location 30 of the gap 28. Since all other cross points for allowing the input waveguide 34 to communicate with the output waveguide 44 are in a transmissive state, a signal that is generated at input waveguide 34 will be received at output waveguide 44. In like manner, the input waveguide 36 is optically coupled to the first output waveguide 42, the third input waveguide 38 is optically coupled to the fourth output waveguide 48, and the fourth input waveguide 40 is optically coupled to the third output waveguide 46.
One concern with optical switching elements 10 of this type is that in the transmissive state, there is a small but potentially objectionable amount of reflection. If the index of refraction of the fluid is different than that of the core material 16, reflections occur. A precise match between the indices of refraction is problematic, since there are other considerations in the selection of a fluid. For example, since the fluid is manipulated using thermal energy, the thermal properties of the liquid must be considered. The greater the mismatch between the index of refraction of the fluid and the index of refraction of the core material 16, the greater the intensity of leakage to the second output waveguide 22 when the switching element is in the transmissive state for optically coupling the collinear waveguides 20 and 26. This leakage will cause crosstalk among the waveguides.
What is needed is a switching arrangement that achieves greater isolation among waveguides of an optical switch. Particularly, what is needed is a switching arrangement that inhibits crosstalk among waveguides.